VERTEBRATE FAUNA OP THE LOUP PORK BEDS. 21 



History. Dr. Leidy did not define this genus until 1869, several years 

 after he had named it. At an earlier period, and in the same paper in 

 in which the name Protohippus was first used. Dr. Leidy named 

 and described a supposed genus MerycUppus, of which the type was 

 M. insignia. Leidy afterwards questioned the distinctness of the two sup- 

 posed genera, and placed the name Protohippus first in joining the two. 

 Coinciding with this view, I place the name Merychippus in the position 

 of synonym to Protohippus, although the name first mentioned was the 

 one first published with a descriptioa. This course I believe justifiable 

 when the names are those of the same author. In this instance the very 

 objectionable name Merychippus has been suppressed with the apparent 

 concurrence of the proposer. 



So far as the characters go, those of the superior molars were pointed 

 out by Dr. Leidy. As these are not sufficient to distinguish the genus 

 from Hippidium, Dr. Leidy at a later date combined the two genera. 

 My discovery in 1873 that Protohippus possesses three toes like Hippothe- 

 rium proved its claim to distinction to be valid. 



This genus is represented by a good many forms, which have an extensive 

 range in size, from the P. placidus, said by Marsh to be two and a half 

 feet in height, to P. mirabilis, Leidy, of about the size of a quagga. 

 Specific differences are also seen in the form of the anteorbital part of the 

 maxillary bone, which presents in some of the species one or more fossae 

 of various size and position. The forms of the teeth are much alike, and 

 the enamel plates present complications in but few species, and then to a 

 less degree than is found in various species of Hippotherium. 



Important changes take place in the pattern of the molar teeth with the 

 advance of age and prolonged use. The folds of the enamel disappear, 

 so that the borders of the lakes which are plicate in the young become 

 perfectly simple in old animals. The period at which the folds disappear 

 differs in different species, coming earlier in the larger species as a rule. 

 The time of simplification differs also in different individuals of the same 

 species, as for instance in the P. placidus (see under that species below). 

 In some of the species the anterior inner column (protocone) remains dis- 

 tinct from the anterior inner crescent (paraconule) for a longer time than 

 in others. In the P. placidus and P. lenticularis the union does not take 

 place until half of the superior molar is worn away. This gave rise to the 

 supposition that the younger individuals of P. placidus represent a species 

 of Hippotherium, which was named H. gratum by Leidy, who, however, 

 expressed doubts as to its validity. I have examined the specimens of 

 the various species of Hippotherium in my collection, with a view of 

 ascertaining whether any others would enter the genus Protohippus on 

 continued wear, but I have failed to detect any except in the cases of 

 isolated teeth. This occurs sometimes in molars of species of the group 



